The differential localization of informational molecules (detemrinants) has been shown to play an important role in restricting early embryonic cells to particular developmental pathways (determination). The germ plasm of Drosophila melanogaster provides one with a unique opportunity to study the molecular nature of these early determinative events. A variety of experiments have established a correlation between localized cytoplasmic components and germ cell determination. However, the molecular nature of these cytoplasmic factors, the mechanism of localization within the ooplasm and their mode of action in development are unknown. The properties of a newly isolated strict maternal effect grandchildless mutation, tudor (tud), indicate that the gene product of the tud locus is required during oogenesis for the determination and/or formation of germ cells and for embryonic abdominal segmentation. In particular, tud+ activity is required for assembly of the germ plasm. This proposal is directed to (1) the identification and characterization of the tud locus at the molecular level, (2) the genetic analysis of the tud locus environs, (3) establishing the distribution of tud transcripts and proteins in oocytes and embryos by molecular and immunological techniques, respectively and (4) the genetic isolation and developmental analysis of new strict maternal effect grandchildless loci required for germ cell determination.